It is most desirable to decrease the radiation doses to which the personnel at nuclear plants is subjected. A major part of these radiation doses is absorbed during the execution of overhaul, service and reparation operations when the nuclear plant is shut off, the personnel among other things being subjected to radiation doses while working with pumps, conduits and the like of a reactor water circuit outside the reactor core. The reason for this is that radioactive corrosion products are deposited on surfaces of system parts outside the very core. Co-60 stands for an absolutely major part of the radioactive radiation from these corrosion products. Furthermore, it has a half-life of 5.3 years, which means that it is not practically possible to decrease the level of the radioactive radiation by letting the personnel execute the work after the reactor has been shut off for a certain period.
In the reactor water circuit and a feed water circuit water causes precipitation of small amounts of material from the different components with which it comes into contact. A major part of these components are made of stainless steel from which iron, nickel and small amounts of cobalt are precipitated as ions and particles. In older nuclear plants certain components in the reactor and feed water circuits, such as valves, contain cobalt, which increases the amount of precipitated cobalt. The metals precipitated in the reactor water and the feed water are deposited in the shape of oxides, such as so called "crud", onto surfaces in the circuit. The crud-surfacing exists as different types of metal oxides, and these, as they for instance are located on cladding tubes for nuclear fuel, are subjected to a strong neutron radiation. Thereby, the metal atoms in the crud surfacing are transformed into isotopes, a part of these being radioactive. Particles fall off and ions are precipitated from the radioactive crud-surfacing and are in that way transferred to the water. Thereby, the particles and the ions are transported together with the reactor water to parts located outside the core and thereby spread radioactive material to those parts. The radioactive particles and the ions are then deposited as a secondary deposited crud-surfacing onto surfaces outside the core. Accordingly, a radioactive crud-surfacing is also produced outside the core, and it is this crud-surfacing that results in the personnel being subjected to radiation doses during service and reparation operations.
In order to restrain the formation of radioactive corrosion products on surfaces of system parts outside the core several, amongst other the following, approaches have been used. According to one approach the crud-surfacing on the fuel is affected such that it is transformed to oxide structures with low solubility, for example a spinel structure. Thereby, the reaction (I) may for instance be used EQU (Ni,Co)O+Fe.sub.2 O.sub.3 .fwdarw.(Ni,Co)Fe.sub.2 O.sub.4 (I)
where (Ni,Co)O indicates nickel oxide contaminated with cobalt, Fe.sub.2 O.sub.3 indicates hematite, and (Ni,Co)Fe.sub.2 O.sub.4 indicates structures of a spinel type, the two first mentioned having a higher solubility in water than the last one during operation conditions. To bring the reaction generally totally to the right, an excess of iron is secured by means of different methods. Thereby, substantially all cobalt ions will make part of the spinel structures, and thereby they will have a relatively low solubility in water, something that substantially decreases the spreading of radioactive corrosion products to system parts located outside the core. According to another approach, the cobalt ions and the cobalt atoms in the oxide surfacing are subjected to competition through an addition of zinc ions to the reactor water. The zinc and cobalt ions are then competing for the same seats in the oxide structures. Thereby, the zinc ions push the cobalt ions aside, and, therefore, Co60 is not absorbed so rapidly on the surfaces of the system parts. The cobalt ions in the water may then be removed during the usual continuous reactor water purification.
The reason for the possible shortage of iron (III) oxide in the reactor water during the reaction (I) is that the continuous condensate purification sometimes is too effective as to purification of iron. The striving for purifying the water as much as possible results in too large amounts of iron being removed, which results in an iron deficit in the reactor water. Therefore, according to Japanese prior art a part of the condensate is conducted past the purification plant in order to, in that way, decrease the amount of iron (III) oxide removed during purification. This method has the important disadvantage of also resulting in an increased amount of cobolt ions in the reactor water.
Another Japanese method according to prior art adds iron (III) oxide to the reactor water through anodic dissolution of iron.
Another method according to prior art adds iron (III) ions in the shape of iron (III) oxalate. The oxalate ion is thereby decomposed in the reactor. The method has the advantage that iron (III) oxalate is soluble in water while a decomposition of the oxalate ion only results in carbon dioxide and water, the carbon dioxide leaving the water in the shape of gas together with steam. A disadvantage is that, during the decomposition of the oxalate ion, carbonic acid is formed, which lowers the pH-value and thereby may result in attacks onto the construction material and the crud-surfacing. Furthermore, the oxalate ion is not decomposed momentarily, but remains a short time in the reactor water, and it is possible that, during this time, it may create problems concerning spreading of radioactivity and corrosion.
Prior art concerning addition of zinc ions to the reactor water or feed water in a nuclear plant provides a method where the zinc ions are added in the shape of organic salts and where the organic counter-ion is decomposed to gaseous products and water in the reactor. Just like the addition of iron in the shape of iron (III) oxalate, this method has the disadvantage of the counter-ion existing in the cooling water during a time period before it is decomposed, and organic acids and finally carbonic acid being formed during the decomposition.
Accordingly, prior art does not provide any accessible method that is substantially free from secondary effects and effective in preventing the deposition of radioactive corrosion products onto surfaces of system parts located outside the core.